Hitherto, a countercurrent continuous contact scheme showing a high contact efficiency has been recognized to be advantageous as a method for solid-liquid contact treatment, i.e., a contact treatment between a solid or solid particles in a slurry and a treatment liquid. In order to effect a uniform and high-efficiency treatment with a small amount of solid-liquid contact, it is desirable to remove a dead zone or a short path for respective streams and improve the solid-liquid mixing so as to promote the renewal of solid-liquid boundary. On the other hand, however, a better mixing is liable to be accompanied with back mixing in the direction of solid and liquid flow axes which remarkably deteriorate the contact efficiency, so that it is difficult to attain a good compatibility there between. In order to reduce the back mixing while maintaining a good solid-liquid mixing state, there has been known a method of partitioning a flow path in a chamber with partitioning plates into a plurality of chambers forming multiple stages, but this cannot provide a good contact efficiency as expected since back mixing is also caused by countercurrent flows between the respective chambers. It is also effective for reducing the back mixing to reduce the sectional area of the flow path between the respective chambers, but this is accompanied with a reduction of treatment capacity and is therefore not practical.
In order to improve the above-mentioned problem, there has been widely and generally known a mixer-settler-type extraction apparatus which includes in separation a mixer section for effecting a sufficient contact and a settler section for uniformly maintaining the respective countercurrent flows, but this requires a large size of apparatus since the functionally-separated respective sections have to retain necessary volumes. Many proposals have been introduced for reducing the apparatus volume, e.g., by adopting vertically arranged multiple stages as disclosed in Patent document 1 listed below. However, according to this type of apparatus, the flows in the settler section are liable to cause a non-uniform portion, and as a result, it becomes difficult to uniformize the treatment on the solid side, so that this type of apparatus is unsuitable as apparatus for operations, particularly for providing an objective product on the solid side, such as washing and impregnation.
In addition to the above, for solid-liquid extraction operation, there has been generally adopted a type of apparatus including a conveyer, such as a belt, baskets or a screw for forming a solid moving layer, and moving a liquid as a counter-current flow on a crossing stream respectively penetrating through the solid moving layer, but a uniform treatment on the solid side is difficult thereby, leaving a problem as an apparatus particularly for operations, such as washing and impregnation, for providing a solid objective product.
As a means for preventing a dead zone and a short path in an apparatus, Patent document 2 listed below discloses to provide a vertically movable stirring blade in each of multi-stage vessels, but on the other hand, no particular attention has been paid for reducing the back mixing.
Further, Patent documents 3 to 5 listed below disclose multi-stage stirring chamber-type apparatus wherein inter-chamber openings are formed between annular partitioning plates and a stirring shaft equipped with stirring blades or disks or between annular partitioning plates and rotating disks affixed to a stirring shaft, and the openings are caused to have a certain thickness in the shaft direction so as to prevent the back mixing in the axial direction. However, all of these apparatus have adopted a form of obstructing inter-vessel streams, so that they may be categorized as apparatus for preventing the back mixing at the cost of a treatment capacity.
As described above, few studies have been made so far for providing a solid-liquid contact apparatus allowing a commercial scale use which allows a good solid-liquid mixing so as to effect a uniform and high-efficiency solid-liquid contact, while reducing the back mixing to prevent a lowering in treatment capacity.    Patent document 1: JP-B 54-12265,    Patent document 2: JP-B 36-13059,    Patent document 3: JP-B 49-41029,    Patent document 4: JP-B 50-8713,    Patent document 5: JP-B 51-18903,